The present invention relates to a method for the underground gasification of solid fuels in which the underground fuel is initially opened up and then converted into a gaseous fuel by means of a chemical reaction with a gasification medium.
It is known that solid fuels, particularly coal, can be gasified at the location where they exist so that mechanical conveyance of the fuel becomes superfluous and less minable fuel deposits can be utilized. In the known underground gasification processes, bore holes are drilled from the earth's surface down to the location of the fuel deposits. Through these bore holes the fuel deposit is opened up by means of a suitable process in order to increase the gas permeability of the fuel which already exists to a greater or lesser degree. Thereafter, the gasification medium is introduced into the opened up fuel deposit through one or a plurality of bore holes and the gasification reaction is started by ignition. Air, oxygen enriched air or air mixed with water vapor can be used as the gasification medium. Underground gasification involves the known gasification reactions listed below:
C+O.sub.2 =CO.sub.2 +97.0 kcal PA0 C+1/2 O.sub.2 =CO+29.3 kcal PA0 C+CO.sub.2 =2CO-38.4 kcal PA0 CO+1/2 O.sub.2 =CO.sub.2 +68.2 kcal PA0 C+H.sub.2 O=CO+H.sub.2 -28.3 kcal PA0 CO+H.sub.2 O=CO.sub.2 +H.sub.2 +10.11 kcal
The gas produced during the underground gasification has a heat value, if 60% oxygen and 40% hydrogen are used as a gasification medium, of about 1350 kcal/Nm.sup.3. This gas is transported out of the fuel deposit through the bore holes and can be utilized as heating gas or, after suitable pretreatment, as synthesis gas.
The opening up of the fuel deposit before the actual underground gasification is necessary to make the fuel deposit sufficiently permeable for the gasification medium and for the resulting gas produced by the gasification. The following known opening up processes have been used for the opening up of the fuel deposit:
(1) The resistance process: in this process electrodes are introduced into the bore holes and a current is applied to the electrodes to heat the fuel deposit and create coked zones in the fuel which are permeable for gases.
(2) The channel combustion process: in this process channels are burnt into the fuel deposit.
(3) Hydraulic bore hole treatment: in this process cracks are formed in the fuel deposit by fluids that are pressed in.
(4) Direct drilling: in this process, bore holes which branch out from the vertical bore holes are driven into the fuel deposit until they reach the next vertical bore hole. Thereafter, the branch bore hole can be widened by burning.
The known opening up processes suffer from the drawback that the volatile organic components in particular, which are present in the solid fuels, cannot be removed. As a result, during the actual underground gasification, the volatile components are driven out of the gasified section of the fuel deposit and clog up the gas permeable pores and cracks existing in the adjacent section of the fuel deposit. Moreover, the water present in the solid fuel is not removed by the prior art opening up processes, with the result that the heating value of the gas generated by the underground gasification is reduced correspondingly.
German Auslegeschrift DE-AS No. 1,493,190 discloses a method for separating mixtures of organic substances by treating the mixtures of organic substances with supercritical gas and subsequently separating the substances dissolved in the resulting supercritical gas phase by reduction of pressure and/or increase in temperature. This publication does not contain any disclosure relating to opening up solid fuel deposits underground by using supercritical gases. Moreover, this publication does not suggest the use of supercritical gas for the underground gasification of solid fuels as an opening up agent, since it could not be expected that particularly the volatile organic compounds could be extracted from the solid fuel in an advantageous manner while still underground and then recovered above ground.